1. Field of the Invention
The present invention relates to improvement of a group of phosphor particles utilized for a light-emitting device, and more specifically to improvement of luminous efficiency and color rendering properties (or color reproducibility) of a light-emitting device utilizing a group of phosphor particles containing a plurality of types of luminescent materials having different fluorescent peak wavelengths.
2. Description of the Background Art
A white light-emitting device formed by combining a semiconductor light-emitting element and luminescent materials is noted as the next-generation light-emitting device expected to have low power consumption, a small size, high brightness, wide-ranging color reproducibility and high color rendering properties, and is actively researched and developed.
In such a white light-emitting device, light having a wavelength in the range of the longer wavelength side of ultraviolet light to blue, i.e., about 380 nm to 480 nm, is employed as primary light emitted from the semiconductor light-emitting element. There is also proposed a luminescence converter utilizing various types of luminescent materials suitable for converting this primary light to secondary light.
Conventionally, it is usual for this type of white light-emitting device to mainly use a combination of a semiconductor light-emitting element for emitting blue light (peak wavelength: about 460 nm) and a trivalent-cerium-activated (Y,Gd)3(Al,Ga)5O12 luminescent material or a divalent-europium-activated 2(Sr,Ba,Ca)O.SiO2 luminescent material to be excited by the blue light so as to emit yellow light.
However, the general color rendering index (Ra) related to such a conventional white light-emitting device is about 70, and particularly the special color rendering index (R9) indicating the degree of rendering of red is at an extremely inferior level of about −40 in the present circumstances. In other words, the conventional white light-emitting device is excessively unsuitable for a general illumination source. The definition and significance of the general and special color rendering indices are specified in JIS also.
Further, the conventional white light-emitting device has insufficient color reproducibility of about 70% according to the NTSC ratio. Nowadays improvement of color reproducibility is demanded also in a small-sized LCD (liquid crystal display) (LCD of a portable telephone, for example). The chromaticity coordinates (x, y) of red, green and blue are (0.670, 0.330), (0.210, 0.710) and (0.140, 0.080) respectively in the chromaticity diagram of XYZ color coordinate system defined by NTSC (National Television System Committee), and the NTSC ratio denotes the ratio to the area of a triangle obtained by connecting these chromaticity coordinates of red, green and blue.
In other words, the white light-emitting device formed by combining the semiconductor light-emitting element and the luminescent materials must be improved in color rendering properties, in the case that it is used as an illumination source. Further, the white light-emitting device must be improved also in color reproducibility (NTSC ratio), in the case that it is used as a backlight for a medium- or small-sized LCD.
For example, Japanese Patent Laying-Open No. 2002-060747 discloses an invention related to color rendering properties of a white light-emitting device formed by combining a semiconductor light-emitting element and luminescent materials. In relation to the white light-emitting device, this Patent Document describes that a general color rendering index (Ra) of 70 to 90 can be obtained when SrGa2S4:Eu2+ and SrS:Eu2+ are employed mainly as green and red luminescent materials, respectively.
However, the thiogallate (SrGa2S4:Eu2+) and the sulfide (SrS:Eu2+) are chemically instable, and particularly the sulfide is easily decomposed when irradiated with ultraviolet light.
Japanese Patent Laying-Open No. 2003-321675 describes in relation to a white light-emitting device that a general color rendering index (Ra) of 75 to 95 can be obtained by utilizing a luminescent material YAG:Ce for emitting yellow light and a nitride luminescent material (general formula: LxMyN(2/3x+4/3y)) such as Ca1.97Si5N8:Eu0.03 for emitting red light and that reddish white light can also be obtained by increasing the value of a special color rendering index (R9) related to red.
When a semiconductor light-emitting element for emitting blue light is combined with the yellow luminescent material YAG:Ce and the red luminescent material Ca1.97Si5N8:Eu0.03 which is a nitride activated with divalent Eu so as to form the white light-emitting device, however, it is difficult to stably obtain a high general color rendering index (Ra) due to a poor luminous component in the green region, and the brightness of the light-emitting device is remarkably reduced due to addition of the red luminescent material (Ca1.97Si5N8:Eu0.03).
An invention related to color reproducibility (NTSC ratio) of an LCD is disclosed in Japanese Patent Laying-Open No. 2003-121838, for example. This Patent Document describes that a backlight source has a spectral peak in the wavelength range of 505 to 535 nm and that a green luminescent material utilized for the backlight source contains any of europium, tungsten, tin, antimony, and manganese as an activator, specifically showing MgGa2O4:Mn and Zn2SiO4:Mn as the green luminescent material in embodiments of the invention.
In the case that the emission peak wavelength of a semiconductor light-emitting element is in the range of 430 to 480 nm, however, not all luminescent materials containing any of europium, tungsten, tin, antimony and manganese are preferably applicable. More specifically, MgGa2O4:Mn and Zn2SiO4:Mn in the embodiments shown in Japanese Patent Laying-Open No. 2003-121838 cannot provide high luminous efficiency with excitation light in the wave range of 430 to 480 nm.
Japanese Patent Laying-Open No. 2004-287323 describes that an RGB (red-green-blue)-LED formed by packaging red, green and blue LED (light-emitting diode) chips, a three-band-type fluorescent lamp, a combination of an ultraviolet LED and an RGB luminescent material, or an organic EL light source is usable as a backlight for an LCD. However, this Patent Document includes no specific description as to a red or green luminescent material suitable for blue excitation light.
U.S. Pat. No. 7,345,317 describes that a light-emitting device can emit light of a stable color regardless of fluctuation in temperature or moisture when phosphor particles contained therein are so limited as to have a particle size of not more than 20 μm and a median diameter (d50) of not more than 5 μm. The median diameter denotes the particle size at a 50% position of particle size distribution. U.S. Pat. No. 6,812,500 describes that the mean particle size of an inorganic luminescent material contained in a light-emitting device is about 10 μm. Japanese Patent Laying-Open No. 2004-210921 describes that the particle size of phosphor particles contained in a light-emitting device is preferably in the range of 1 to 20 μm, more preferably in the range of 2 to 8 μm (according to air permeation). However, none of U.S. Pat. No. 7,345,317, U.S. Pat. No. 6,812,500 and Japanese Patent Laying-Open No. 2004-210921 describes the mutual relation between the particle sizes of a plurality of types of phosphor particles contained in a light-emitting device.
Japanese Patent Laying-Open No. 2007-049114 describes that at least one of a plurality of types of phosphor particles has an absorption band capable of absorbing light emitted from at least another type of luminescent material; a phosphor particle species for emitting light having a relatively long wavelength is distributed closer to a semiconductor light-emitting element; red light-emitting particles are divalent-europium-activated nitride phosphor particles, expressed in a general formula (MI1-aEua)MIISiN3, capable of emitting red light by absorbing green light (wavelength: about 520 nm) and blue light (wavelength: about 450 nm) emitted from green and blue phosphor particles respectively; the plurality of types of phosphor particles have different median diameters; and liquid resin containing the plurality of types of phosphor particles is allowed to stand for a prescribed time for settling a phosphor particle species of a larger median diameter and increasing the distribution density thereof in the vicinity of the semiconductor light-emitting element; and further describes in relation to Embodiment 16 that transparent resin contains large-, medium- and small-sized phosphor particles, these phosphor particles having different particle sizes are separated substantially in a layered manner, and the median diameters of red, green and blue phosphor particles are 13 μm, 9.5 μm and 6.5 μm, respectively.
However, this Patent Document discloses a plurality of phosphor particles having different median diameters only in relation to Example 16, and there is no mention regarding control of median diameters in consideration of practical factors such as brightness and workability in relation to various types of phosphor particles.